The solar nebula theory—which states that the solar system formed from a vast cloud of gas and dust more than 4.5 billion years ago—is well established. Over the course of hundreds of millions of years, the cloud contracted and matter accreted (came together) to form small bodies, which collided with each other to eventually form the planets, moons, and other solar system objects. The same basic chemical ingredients were available throughout the cloud, but because different materials condense at different temperatures, various types of objects formed depending on how close they were to the Sun.

Earth is the only planet currently known to support life. How did the evolution of the solar system lead to Earth's water-rich, habitable environment? All the inner planets formed from metals and rocky materials and lacked the kinds of atmospheres we see today. But Earth developed a stable atmosphere plus oceans that cover 71 percent of its surface; the presence of liquid water and the composition of the atmosphere led to conditions hospitable for life to evolve. In contrast, Mercury is a dry, barren world very close to the Sun. It is too small and too hot to retain any substantial atmosphere, and its surface temperatures range from hundreds of degrees above to hundreds of degrees below freezing. Venus, closer to the Sun than Earth, is also too hot for liquid water; it has a thick carbon dioxide atmosphere and surface temperatures of about 430°C (800°F). Mars, farther from the Sun than Earth, has a very thin atmosphere of carbon dioxide and is too cold to have liquid water on its surface.

How Earth developed its oceans and atmosphere is an active area of research. There are several hypotheses and it is likely that there was more than one contributor to Earth's supply of water. One popular hypothesis is called outgassing, a phenomenon in which a gas is released from a material, such as during a tremendous impact. Water was abundant in the solar nebula—comets and asteroids are made up of mostly ice and some rocky materials—so it is likely that water was also present in the material that accreted to form Earth. A major collision could have heated the young Earth so much that it released, or outgassed, the water vapor that was trapped within its rocks, which could have formed an atmosphere around the planet. Similarly, volcanic activity could have released gases (including water vapor) from Earth's interior into the atmosphere. As the planet cooled, water vapor condensed into water droplets, which fell to the surface as rain and gradually collected into the oceans.

Another popular hypothesis is delivery by impacts from comets and asteroids. It is possible that during a period known as the Late Heavy Bombardment, after major collisions had occurred between protoplanets, the inner solar system was battered by thousands of comets and asteroids that delivered water and other organic compounds with every impact. Craters on moons and planets are evidence of such impacts from asteroids and comets.

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